Abstract. We present X-ray spectral fits to a recently obtained Chandra grating spectrum of η Carinae, one of the most massive and powerful stars in the Galaxy and which is strongly suspected to be a colliding wind binary system. Hydrodynamic models of colliding winds are used to generate synthetic X-ray spectra for a range of mass-loss rates and wind velocities. They are then fitted against newly acquired Chandra grating data. We find that due to the low velocity of the primary wind (≈500 km s −1 ), most of the observed X-ray emission appears to arise from the shocked wind of the companion star. We use the duration of the lightcurve minimum to fix the wind momentum ratio at η = 0.2. We are then able to obtain a good fit to the data by varying the mass-loss rate of the companion and the terminal velocity of its wind. We find thatṀ 2 ≈ 10 −5 M yr −1 and v∞ 2 ≈ 3000 km s −1 . With observationally determined values of ≈500-700 km s −1 for the velocity of the primary wind, our fit implies a primary mass-loss rate ofṀ 1 ≈ 2.5 × 10 −4 M yr −1 . This value is smaller than commonly inferred, although we note that a lower mass-loss rate can reduce some of the problems noted by Hillier et al. (2001) when a value as high as 10 −3 M yr −1 is used. The wind parameters of the companion are indicative of a massive star which may or may not be evolved. The line strengths appear to show slightly sub-solar abundances, although this needs further confirmation. Based on the over-estimation of the X-ray line strengths in our model, and re-interpretation of the HST/FOS results, it appears that the Homunculus nebula was produced by the primary star.

We use 3D hydrodynamical models to investigate the effects of massive star feedback from winds and supernovae on inhomogeneous molecular material left over from the formation of a massive stellar cluster. We simulate the interaction of the mechanical energy input from a cluster with 3 O-stars into a giant molecular cloud (GMC) clump containing 3240 M of molecular material within a 4 pc radius. The cluster wind blows out of the molecular clump along low-density channels, into which denser clump material is entrained. We find that the densest molecular regions are surprisingly resistant to ablation by the cluster wind, in part due to shielding by other dense regions closer to the cluster. Nonetheless, molecular material is gradually removed by the cluster wind during which mass-loading factors in excess of several 100 are obtained. Because the clump is very porous, 60 − 75 per cent of the injected wind energy escapes the simulation domain, with the difference being radiated. After 4.4 Myr, the massive stars in our simulation begin to explode as supernovae. The highly structured environment into which the SN energy is released allows even weaker coupling to the remaining dense material and practically all of the SN energy reaches the wider environment. The molecular material is almost completely dispersed and destroyed after 6 Myr. The escape fraction of ionizing radiation is estimated to be about 50 per cent during the first 4 Myr of the cluster's life. A similar model with a larger and more massive GMC clump reveals the same general picture, though more time is needed for it to be destroyed.

We report the results of an observing campaign on Car around the 2003 X-ray minimum, mainly using the XMMNewton observatory. These are the first spatially resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, associated with the wind-wind collision (WWC) in the binary system, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The X-ray flux in the 2Y10 keV band seen by XMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5Y10 to N H $ (3Y 4) ; 10 23 cm À2 . These variations were qualitatively consistent with emission from the WWC plasma entering into the dense wind of the massive primary star. During the minimum, X-ray spectra also showed significant excesses in the thermal Fe xxv emission line on the red side, while they showed only a factor of 2 increase in equivalent width of the Fe fluorescence line at 6.4 keV. These features are not fully consistent with the eclipse of the X-ray plasma and may suggest an intrinsic fading of the X-ray emissivity. The drop in the WWC emission revealed the presence of an additional X-ray component that exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high-speed outflows at v $ 1000Y2000 km s À1 from Car with ambient gas within a few thousand AU from the star.

The X‐ray emission from the supermassive star η Car is simulated using a 3D model of the wind–wind collision. In the model the intrinsic X‐ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled post‐shock material from the primary LBV star is calculated as the intrinsic emission is ray traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X‐ray data, including the light curve observed by the Rossi X‐ray Timing Explorer (RXTE) and spectra observed by XMM–Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ≃42°, consistent with the polar axis of the Homunculus nebula, and the projection of the observer's line of sight on to the orbital plane has an angle of ≃0°–30° in the prograde direction on the apastron side of the semimajor axis. However, there are significant discrepancies between the observed and model light curves and spectra through the X‐ray minimum. In particular, the hard flux in our synthetic spectra is an order of magnitude greater than observed. This suggests that the hard X‐ray emission near the apex of the wind–wind collision region (WCR) ‘switches off’ from periastron until two months afterwards. Further calculations reveal that radiative inhibition significantly reduces the pre‐shock velocity of the companion wind. As a consequence the hard X‐ray emission is quenched, but it is unclear whether the long duration of the minimum is due solely to this mechanism alone. For instance, it is possible that the collapse of the WCR on to the surface of the companion star, which would be aided by significant inhibition of the companion wind, could cause an extended minimum as the companion wind struggles to re‐establish itself as the stars recede. For orbital eccentricities, e≲ 0.95, radiative braking prevents a wind collision with the companion star's surface. Models incorporating a collapse/disruption of the WCR and/or reduced pre‐shock companion wind velocities bring the predicted emission and the observations into much better agreement.

Three dimensional (3D) adaptive-mesh refinement (AMR) hydrodynamical simulations of the windwind collision between the enigmatic super-massive star η Car and its mysterious companion star are presented which include radiative driving of the stellar winds, gravity, optically-thin radiative cooling, and orbital motion. Simulations with static stars with a periastron passage separation reveal that the preshock companion star's wind speed is sufficiently reduced that radiative cooling in the postshock gas becomes important, permitting the runaway growth of non-linear thin shell (NTSI) instabilities which massively distort the WCR. However, large-scale simulations which include the orbital motion of the stars, show that orbital motion reduces the impact of radiative inhibition, and thus increases the acquired preshock velocities. As such, the postshock gas temperature and cooling time see a commensurate increase, and sufficient gas pressure is preserved to stabilize the WCR against catastrophic instability growth. We then compute synthetic X-ray spectra and lightcurves and find that, compared to previous models, the X-ray spectra agree much better with XMM-Newton observations just prior to periastron. The narrow width of the 2009 X-ray minimum can also be reproduced. However, the models fail to reproduce the extended X-ray mimimum from previous cycles. We conclude that the key to explaining the extended X-ray minimum is the rate of cooling of the companion star's postshock wind. If cooling is rapid then powerful NTSIs will heavily disrupt the WCR. Radiative inhibition of the companion star's preshock wind, albeit with a stronger radiationwind coupling than explored in this work, could be an effective trigger.